This invention relates to the defluorination of fumed silica. More particularly, this invention relates to a process for the defluorination of fumed silica made in a fluoride process wherein the fumed silica is substanttially uniformly entrained in a stream of steam at a temperature above about 1200.degree. F. and, thereafter, is separated from the steam to obtain a fumed silica product with a substantially reduced fluoride content.
Fumed silica, made in a fluoride process by the flame hydrolysis of silicon tetrafluoride, contains between two and four percent fluoride. This fluoride-rich silica has hydrophobic properties and is subject to a substantial loss of surface area under ordinary storage conditions due to a chemical reaction of the fluoride with the silica. When the fluoride content of the silica is reduced to half a percent or less, the silica becomes hydrophilic, and its surface area remains stable under ordinary storage conditions.
It is known that fumed silica may be defluorinated by thermal means alone, but high temperatures and long residence times are necessary to achieve satisfactory results. Lower temperatures and shorter residence times can be achieved by the addition of steam in the process. Lower temperatures are also beneficial in reducing the probability of sintering. The presence of steam ensures that the surface of the silica will include silanol groups, which are thought to be necessary for the hydrophilic, thickening and thixotropic properties of fumed silica. This invention provides a more efficient steam process for the defluorination of fumed silica than exists at the present time.